Evidence for dopaminergic regulation of prolactin and a luteotropic complex in the ferret

Reproductive features and faecal progesterone metabolite profile in female ferrets

Elevated post-partum progesterone metabolite (P(4) -met) levels have been recently postulated to occur in lactating lynxes. The aims of this study were to monitor reproductive features in female ferrets, changes in the faecal P(4) -met concentrations throughout the breeding season and ovarian activity in post-partum lactating and non-lactating (NL) female ferrets. Our results indicate that coinciding with the results described in the lynx, elevated faecal P4-met concentrations occur in lactating ferrets, furthermore, that the duration of elevated secretion of P(4) seems to be dependent on the duration of lactation (P4-met at delivery, n=47: <500 ng/g; 5-7 days after delivery, during lactation, n=47: ≥ 500-800 ng/g; in females weaned at delivery, n=4: baseline levels). Three days after ovariohysterectomy of lactating females, P(4) -met concentrations decreased to baseline levels. In lactating females, the ovarian stroma is more active than that in NL ones implicating that the ovary is at least in part responsible for the elevated P4-met concentrations. Ovaries of lactating females contained many luteinized cells either as luteinized granulose cells in the wall of late pre-antral/early antral follicles or as corpus luteum (CL)-like structures. Early resumption of the entire ovarian activity (developed follicles and oestrus) occurs in NL post-partum females, while final follicular development is blocked (follicles stalls at antral stage) in the lactating ones (however, occasionally lactational oestrus may occur). We suppose that the elevated faecal P4-met during lactation together with suckling and other hormonal effects may contribute to prevention of early returning to oestrus in nursing female ferrets.

Reproduction of the female ferret (Mustela putorius furo)

The domestic ferret is a seasonally polyoestrous species. Females reach puberty at the age of 8-12 months. Females exhibit a constant oestrus between late March and early August if they are not bred. Increasing tumescence in the pink-coloured vulva is a sign of pro-oestrus. Oestrus can persist for up to 5 months, but once ovulation is induced, either pregnancy or pseudopregnancy ensues. Follicular development and atresia overlap in such a manner that there is a recent cohort of follicles available for ovulation whenever copulation might occur. Copulation may last from 15 min to 3 h, the average being 1 h. Ovulation is induced by pressure on the cervix associated with copulation. After sufficient LH release, the pre-ovulatory follicles mature and an average of 12 oocytes (5-13) per female are ovulated 30-40 h after copulation into the ovarian bursa. The ferret oocytes are most capable of being fertilized up to 12 h after ovulation, i.e. 42-52 h after copulation. Ferret oocytes ovulate at the metaphase of the second meiotic division (MII) embedded in three layers of corona radiata cells. Embryos enter the uterus over a period of several days starting on day 5 after mating. Between days 12 and 13 after mating, the embryos have become implanted in the endometrium. Implantation in the ferret is central with rapid invasion of the uterine epithelium by the trophoblast over a broad area that eventually becomes a zonary band of endotheliochorial placenta. Gestation length is 41 days (39-42 days). The domestic ferret gives birth to an average of eight kits (1-18 kits), which weigh 6-12 g at birth.

Plasma progesterone and prolactin concentrations in overtly pseudopregnant bitches: A clinical study

Plasma concentrations of progesterone (P(4)) and prolactin (PRL) were measured in 35 bitches presented at veterinary clinics for symptoms of overt pseudopregnancy (PSP) between 50 and 95 days after the onset of proestrus. Results were compared to those from samples collected from 35 control bitches at comparable stages of the ovarian cycle (expressed as days after the onset of observed signs of proestrus). In the PSP bitches at 71.4+/-1.6 (mean+/-S.E.M.) days of the cycle, P(4) (1.5+/-0.2ng/mL) was lower (P<0.01) and PRL (16.0+/-1.9ng/mL) was higher (P<0.01), compared to P(4) (2.7+/-0.4ng/mL) and PRL (2.9+/-0.6ng/mL) in control bitches at 70.6+/-1.5 days of the cycle. Low P(4) was not a prerequisite for elevated PRL. Although elevated (> or =10ng/mL) PRL (20.9+/-2.0ng/mL) occurred more often with low (<2ng/mL) P(4) (20 of 24 cases) it also occurred with P(4) above 3ng/mL in two affected bitches and in two control bitches. Whether the occurrence of relatively low PRL concentrations (<4ng/mL) in samples obtained from 4 of the 35 pseudopregnant bitches reflected variable and often elevated PRL secretion or increased sensitivity to PRL in the absence of elevated prolactin in those animals was not determined. We inferred that elevated plasma PRL was often involved in the etiology of overt PSP; furthermore, a premature decline in circulating P(4) concentrations may be a factor in some instances.

Bait-delivered pimozide causes precocious embryo implantation in mink: a fertility control option for the exotic stoat?

Stoats (Mustela erminea), an exotic pest in New Zealand, threaten the conservation of several ground-nesting bird species and broad-scale methods for their control are sought. Females are seasonally monestrous, show a 9-month period of obligatory diapause and usually do not breed more than once in their lives. A bait-delivered agent that terminates diapause and results in a non-viable embryo may have a significant impact on their reproductive success. Prolactin (PRL) is hypothesised to be the only gonadotrophin required for renewal of luteal activity and blastocyst implantation in some mustelids. We investigated the effects of bait-delivered dopamine (DA) antagonists (which stimulate the release of PRL) using a mink model (Mustela vison), a species that maintains a short period of diapause. A bait dose of 0.8 mg kg(-1) of pimozide was more effective in elevating PRL levels than equivalent doses of fluphenazine, sulpiride (P < 0.01) or haloperidol (P < 0.05). Bait doses of 1.6 mg kg(-1) pimozide given at Days 0, 3, 9 and 11 after mating caused a significant reduction in the length of pregnancy compared with a positive control and placebo (46 days v. 51 days), indicating early termination of diapause (P < 0.01). Pimozide doses caused higher elevations in PRL concentration relative to the oral placebo by Day 12, but mean PRL levels fell below all other groups by Day 18. A borderline significant increase in progesterone (P4) secretion compared with the oral placebo was detected at Day 18. These results suggest that bait-delivered pimozide can elevate PRL outside of the normal breeding season and doses of 1.6 mg kg(-1) are effective in terminating embryonic diapause in mink. The implications and limitations of these data are discussed with reference to the use of bait-delivered DA antagonists as a possible means to affect the reproductive success of wild stoats.

Pituitary hormone responses to luteal regression after administration of prostaglandin F2alpha-analogue

As an experiment to elucidate the canine luteal function maintenance system, a PGF2alpha-analogue, fenprostalene (PGF-F), was administered 24 and 54 days after ovulation to induce luteal regression, and the responses of luteinizing hormone (LH) and prolactin (PRL), which are considered to support the canine corpus luteum, were investigated. The plasma progesterone (P(4)) level was rapidly decreased, by which the plasma PRL level was increased, but no change was observed in the plasma LH level. The decrease in the plasma P(4) level after PGF-F administration may have induced positive feedback to the superior system, and stimulated secretion of the pituitary hormones. These findings suggested that the canine corpus luteum is maintained by PRL, not by LH.

Changes in sex steroids, gonadotropins, prolactin, and inhibin in pregnant and nonpregnant Japanese black bears (Ursus thibetanus japonicus)

We examined changes in the concentrations of serum progesterone (P4), estradiol-17beta (E2), FSH, LH, prolactin (PRL), and inhibin to determine their interaction and their effect on the reproductive endocrine controls of pregnant and nonpregnant female Japanese black bears. Fourteen female bears were used in this study over a 2-yr period. In the first year, six of the bears were divided into two groups; a pseudopregnant group and a nonpregnant group. In the second year, the remaining eight bears were also divided into two groups; a pregnant group and a nonpregnant group. Pregnant and pseudopregnant bears had similar P4 trends with both groups exhibiting a significant increase in December, which is the suspected time of implantation in pregnant bears. These trends correlated with an increase in PRL levels, whereas low levels of LH were maintained throughout the year. Nonpregnant bears maintained low concentrations of P4, and compared with pregnant and pseudopregnant bears, they also exhibited a delayed elevation in PRL. Luteinizing hormone activity varied among individual animals, but regardless of reproductive status, fluctuation patterns of E2, FSH, and inhibin did not differ among bears. Our results suggest that PRL may play a luteotropic role in both pregnant and pseudopregnant bears, and is possibly responsible for inducing reactivation of the dormant corpus luteum that precedes implantation in the Japanese black bear.

Ultrastructural and immunocytochemical studies of prolactin-secreting cells in adenohypophysis of the mink (Mustela vison)

This investigation aimed to identify, by the double immunogold procedure, the ultrastructural characteristics of prolactin (PRL) cells in the mink. Such cells showed a marked pleomorphism and had a close topographic relationship with growth hormone cells. A common morphological characteristic of PRL cells in all stages of mink development was the presence of round secretory granules, in contrast to changes in the ultrastructural characteristics of PRL cells with physiological state and photoperiod. Thus PRL cells in prepubertal, pubertal, and sexually inactive adult mink, killed under a short-day photoperiod, showed little development of the organelles but a significantly increased cytoplasmic electron density. In sexually active mink and in lactating females under long-day conditions, PRL cells had a highly developed cytoplasmic organelle structure consisting mainly of rough endoplasmic reticulum. The morphometric study demonstrated that the mean diameter of the secretory granules similarly varied in both sexes. Pubertal mink had PRL cells with smaller secretory granules (female 74.1 +/- 0.6 nm, male 80.4 +/- 1.7 nm), whereas adult mink killed under a long-day photoperiod presented PRL cells with larger secretory granules (female 194.5 +/- 2.2 nm, male 203.3 +/- 1.7 nm). The changes in the ultrastructural characteristics of PRL cells during the annual cycle suggest a photoperiodic influence upon these cells. In addition the heterogeneity in ultrastructural characteristics and storage characteristics of PRL in some adult mink may suggest a varying metabolic role for PRL under certain, as yet not fully characterized, conditions.

In vivo investigation of luteal function in dogs: effects of cabergoline, a dopamine agonist, and prolactin on progesterone secretion during mid-pregnancy and -diestrus

The role of prolactin on luteal function in dogs was investigated in vivo. The function of prolactin in mid-luteal phase was compared in pregnant and nonpregnant dogs. A dopamine agonist, cabergoline, known for its prolactin secretion inhibitory effects, was injected subcutaneously at a dose of 5 micrograms/kg body weight in five pregnant and five nonpregnant Beagle bitches. Mean plasma prolactin and progesterone were dramatically suppressed for 4 to 5 days after injection in both groups when compared with control pregnant and non-pregnant animals, whereas no effect on luteinizing hormone (LH) secretion was observed. The decline in plasma progesterone occurred after that in prolactin, suggesting plasma progesterone was impaired by inhibition of prolactin secretion. These results confirm the luteotropic importance of prolactin in pregnant bitches, and also demonstrate its importance in luteal phase of the nonpregnant dog. Second, to demonstrate that the effects of cabergoline were mediated by prolactin inhibition and not by a direct action on the corpus luteum, concomitant administration on Day 30 of cabergoline and prolactin (375 micrograms i.v. twice daily on Days 30 and 31) or cabergoline and LH (750 micrograms i.v. twice daily on Days 30 and 31) was affected in two groups of five pregnant animals each. Results showed that only prolactin was able to reverse the negative effects of cabergoline on circulating progesterone. This confirms the indirect mode of action of the dopamine agonist, cabergoline on corpus luteum function. Third, further investigation on the precise luteotropic role of prolactin was made by IV injection of 375 micrograms pure canine prolactin twice daily in five pregnant bitches on Days 30 and 31, and in five pregnant bitches on Days 40 and 41. No direct stimulatory effect of prolactin on plasma progesterone secretion occurred. Nor was there a noticeable effect on plasma LH secretion. These results suggest that prolactin is unable to directly stimulate progesterone secretion by the corpus luteum of pregnancy. The results of this study suggest that prolactin is an essential luteotropin in the dog from mid-luteal phase in both pregnant and nonpregnant animals. However, it appears to act by sustaining corpus luteum lifespan and function rather than by direct stimulatory effects on progesterone secretion.

Seasonal fiber growth cycles of ferrets (Mustela putorius furo) and long-term effects of melatonin treatment

Pelage cycles of ferrets are poorly documented, although it is clear that their timing is sensitive to daylength, mediated by pineal melatonin. Hair follicles were monitored histologically in ferrets from 3 to 19 months of age in order to describe naturally occurring changes in follicle growth status and follicle number over three successive cycles of fur growth. Melatonin was administered to some of these animals in late summer to determine the long-term effects of perturbation of hormonal control. Circulating melatonin was elevated for approximately 50 days by 8-mg continuous release implants. Treated animals grew both their first winter coat, and subsequent summer coats 18 days in advance of untreated controls, but this effect did not extend to the second winter coat. Reimplantation the following year induced an advancement of the autumn follicle growth as in the 1st year. Autumn fiber growth occurred at similar times in untreated males and females, and response to melatonin did not differ between sexes. Hair follicle regression and shedding during the natural spring molt was also contemporaneous in males and females, but fiber regrowth occurred 4-6 weeks later in males as compared with females, suggesting that reproduction-related factors affect fiber growth initiation, and that fiber growth and shedding are physiologically distinct processes. Melatonin implants in autumn also affected reproduction in the spring, advancing oestrus by 3-4 weeks. These results show that interference with photoperiodic and hormonal control mechanisms in ferrets can affect pelage and reproductive cycles for up to 10 months.

Evidence for prolactin as the main luteotrophic factor in the cyclic dog

The role of prolactin and LH in the control of the function of the corpus luteum in the dog was studied. Experiments were performed to interfere with the secretion of a) prolactin by administering a dopamine agonist and b) LH by desensitisation with a long-acting LHRH and by stimulation. Treatments with prolactin-lowering dosages of bromocriptine, (20 micrograms/kg body weight twice a day, orally; n = 8) which started between day 1-5 (n = 4) and day 20-24 (n = 4) of the luteal period resulted in a similar pattern of progesterone, concentration in peripheral blood in both groups. The progesterone release in the second half of the luteal period (13.1 +/- 1.8% (sem) of the progesterone release of the total luteal period) was significantly lower than in control dogs (24.7 +/- 2.2%). Treatment at about day 30 of the luteal period with LHRH CR (1.34 mg, intramuscularly; n = 3), which significantly suppressed the LH level, did not reduce the progesterone release in the second half of the luteal period, 21.3 +/- 4.7% compared to 24.7 +/- 2.2% in the control dogs. The endogenous LH peak resulting from treatment with LHRH had no effect on the progesterone concentration in the blood. It is concluded that prolactin is the main luteotrophic factor in the cyclic dog during the second half of the luteal period.

In vitro binding and utilization of lipoproteins by luteal cells from ferrets treated with dopaminergic drugs during pseudopregnancy

The effects of administration of dopaminergic drugs in vivo on the binding and utilization of lipoproteins for progesterone synthesis in vitro by ferret luteal cells were investigated. Pimozide, a dopamine antagonist, and bromoergocriptine (CB-154), a dopamine agonist, were administered to pseudopregnant ferrets to alter prolactin (PRL) concentrations daily beginning the day after ovulation. The control group received the vehicle solution only. Corpora lutea taken on Day 13 after ovulation were dissociated and the cells were incubated with canine lipoproteins, cyclic adenosine monophosphate (cAMP), and 5-cholesten-3 beta-25-diol (25-OH-cholesterol). Canine high-density lipoprotein (HDL) and low-density lipoprotein (LDL) stimulated progesterone accumulation by luteal cells from pimozide-treated animals but not from CB-154-treated ferrets. However, when 25-OH-cholesterol, which bypasses the LDL receptor, was provided as the substrate, steroidogenesis was stimulated in all groups. Together these observations suggest that dopaminergic alteration of PRL levels preferentially affects the utilization of lipoproteins. The uptake of canine HDL and LDL by luteal cells was saturable, and a high degree of cross-reactivity was observed. Heparin released surface-bound HDL and LDL, suggesting that HDL was binding to the LDL receptor. The quantity of LDL which could be released from luteal cells by heparin treatment was greater in animals treated with pimozide and decreased by treatment with CB-154, relative to luteal cells from control animals. It was concluded that the chronic administration of pimozide or CB-154 alters serum PRL levels in vivo, and influences the subsequent binding and utilization of lipoproteins by luteal cells in vitro. PRL may increase the number of LDL binding sites in luteal cells, thereby enhancing lipoprotein uptake for progesterone synthesis.